Selected Findings and Current Perspectives on Urban and Agricultural
Water Quality by the National Water-Quality Assessment Program

Studies by the USGS National Water-Quality Assessment (NAWQA) program
in the last decade describe water-quality conditions in nearly 120
agricultural and 35 urban watersheds ("urban" primarily
refers to residential and commercial development over the last 50
years). The findings show that for both urban and agricultural areas, nonpoint chemical contamination is an issue. Much work still needs to be done in urban areas with point source contamination as well, including infrastructure improvements. Appreciable improvements in overall water quality, however, will depend upon effective management of point and nonpoint sources. The findings show that nonpoint chemical contamination is an
agricultural and urban issue. Whereas a lot of work still needs to be
pursued with point source contamination and infrastructure
improvements in urban areas (such as related to combined and sanitary
sewer overflows), appreciable improvements in water quality also will
depend upon management of nonpoint sources. The NAWQA findings also
show that water-quality conditions and aquatic health reflect a
complex combination of land and chemical use, land-management
practices, population density and watershed development, and natural
features, such as soils, geology, hydrology, and climate. Contaminant
concentrations vary from season to season and from watershed to
watershed. Even among seemingly similar land uses and sources of
contamination, different areas can have very different degrees of
vulnerability and, therefore, have different rates at which improved
treatment or management can lead to water-quality improvements.

Water Quality in Agricultural Watersheds

Nitrogen and phosphorus in surface water commonly exceed levels that
contribute to excessive algae. For example, average annual
concentrations of phosphorus in nearly 80 percent of streams sampled
in agricultural areas were greater than the U. S. Environmental
Protection Agency (USEPA) desired goal for preventing nuisance plant
growth in streams. Excessive plant growth can lead to low dissolved
oxygen, which can be harmful to fish and other aquatic life.

Nitrate is often elevated above background levels in shallow ground
water underlying farmland. Concentrations in about 20 percent of
shallow wells sampled in agricultural areas exceeded the USEPA
drinking water standard. This result is a concern in rural areas where
shallow ground water is used for domestic supply; these domestic wells
are not regulated and owners often do not know the quality of their
well water or whether their wells are vulnerable to contamination.
Nitrate is most often elevated in karst (carbonate) areas or where
soils and aquifers consist of sand and gravel. These natural features
enable rapid infiltration and downward movement of water and
chemicals. Some of the more vulnerable areas are the Central Valley of
California, and parts of the Pacific Northwest, the Great Plains, and
the Mid-Atlantic region. In contrast, ground-water contaminants
underlying farmland in parts of the upper Midwest are barely
detectable, despite similar high rates of chemical use. In these areas
ground-water contamination may be limited because of relatively
impermeable, poorly drained soils and glacial till that cover much of
the region, and because tile drains provide quick pathways for runoff
to streams.

Pesticides are widespread. At least one pesticide was detected in more
than 95 percent of stream samples. Pesticides were detected in more
than 60 percent of shallow wells sampled in agricultural areas.

Pesticides commonly occur in mixtures. Two-thirds of stream samples
collected in agricultural areas contained 5 or more pesticides, and
more than one-quarter of the samples contained 10 or more. Ground
water contained fewer pesticides; about 30 percent of the wells
sampled contained 2 or more.

Concentrations of pesticides generally are low and below
drinking-water standards. However, the risk to humans and the
environment from present-day low levels of contaminant exposure
remains unclear. For example, current standards and guidelines do not
yet account for exposure to mixtures, and many pesticides and their
breakdown products do not have standards or guidelines.

Herbicidesmost commonly atrazine and its breakdown product
desethylatrazine, and metolachlor, cyanazine, and alachloroccur
more frequently and usually at higher concentrations in agricultural
streams and ground water than in urban waters. Their occurrence is
linked to their use; they rank in the top five in national herbicide
use for agriculture.

Insecticides that were used in the past still persist in agricultural
streams and sediment. DDT was the most commonly detected
organochlorine compound, followed by dieldrin and chlordane. Their
uses were restricted in the 1970s and 1980s and, yet, more than 20
years later, one or more sediment-quality guidelines were exceeded at
more than 20 percent of agricultural sites.

Water Quality in Urban Watersheds

Concentrations of total phosphorus are generally as high in urban
streams as in agricultural streams. More than 70 percent of sampled
urban streams exceeded the USEPA desired goal for preventing nuisance
plant growth.

Insecticides, such as diazinon, carbaryl, chlorpyrifos, and malathion,
occur more frequently, and usually at higher concentrations in urban
streams than in agricultural streams. Concentrations are low in urban
streams, rarely exceeding USEPA drinking-water standards. However,
effects on aquatic life may be more of a concern. Concentrations of
insecticides exceeded at least one guideline established to protect
aquatic life in every sampled urban stream.

Herbicides are widespread in surface water (detected in 99 percent of
urban stream samples) and ground water (detected in more than 50
percent of sampled wells). Most common are those applied to lawns,
golf courses, and road right-of-ways, such as atrazine, simazine, and
prometon.

Similar to agricultural areas, pesticides in urban waters commonly
occur in mixtures; nearly 80 percent of stream samples contained 5 or
more pesticides. Two of the most commonly detected insecticides in
mixtures were diazinon and chlorpyrifos; common herbicides detected
were simazine and prometon.

Sediment in urban streams is associated with higher frequencies of
occurrence of DDT, chlordane, and dieldrin and higher concentrations
of chlordane and dieldrin than sediment in agricultural streams.
Sediment-quality guidelines for organochlorine pesticides were
exceeded at 36 percent of sampled urban sites.

Volatile organic compounds, which are used in plastics, cleaning
solvents, gasoline, and industrial operations, occur widely in shallow
urban ground water. Some of the most frequently detected of the 60
analyzed compounds were the commercial and industrial solvents
trichloroethene (TCE), tetrachloroethene (PCE), and methylene
chloride; the gasoline additive methyl tert-butyl ether (MTBE); and
the solvent and disinfection by-product of water treatment,
trichloromethane (also known as chloroform).

Concentrations of selected trace elements, such as cadmium, lead,
zinc, and mercury, are elevated above background levels in populated
urban settings, most likely caused by emissions from industrial and
municipal activities and motor vehicles. Sediment cores from
streambeds and reservoirs, which can be used to track changes over
long time periods, indicate that lead increased from 1940s to the
1970s, and began to decrease after it was removed from gasoline.
Concentrations are not yet down to background levels. Decreases also
are noted for DDT and chlordane.

In contrast to lead, DDT, and chlordane, sediment cores indicate that
zinc and polycyclic aromatic hydrocarbons (PAHs, which result from
fossil fuel combustion) are increasing. These increases most likely
relate to increasing motor vehicle traffic in watersheds.
Sediment-quality guidelines for PAHs were exceeded at more than 40
percent of urban sites.

Toxic compounds in streambed sediment in urban areas, such as DDT,
chlordane, dieldrin, and PCBs, also were found in fish tissue, often
at higher concentrations than in the sediment. One or more
organochlorine compounds were detected in 97 percent of whole fish samples
collected at urban sites, and PCBs were detected in more than 80
percent of whole fish samples. Concentrations of organochlorine compounds
exceeded guidelines to protect wildlife at more than 10 percent of
urban sites; wildlife guidelines for PCBs were exceeded at nearly 70
percent of urban sites. These findins have contributed to decisions by some states to issue fish-consumption advisories.

Deteriorated water quality and sediment, as well as habitat
disturbances, contribute to degraded biological communities in urban
streams. The greatest effects are seen in areas with the highest human
population densities and watershed development. Pollution-tolerant
algae and aquatic invertebrates (such as worms and midges), as well as
omnivorous fish communities, prevail at the affected sites.